As adhesives, rubber-based adhesives, acrylic adhesives, epoxy-based adhesives, vinyl ester-based adhesives, silicone-based adhesives, etc. have been used in the past. Of these, (meth)acrylic adhesives are excellent in weathering resistance and oxidation deterioration resistance, and they are practically applied to uses in various fields. For example, uses for household articles, such as double-coated tapes, packaging tapes and dust collecting rollers, and industrial uses, such as industrial tapes for assembling or fixing parts of automobiles or household electrical appliances can be mentioned, and the (meth)acrylic adhesives have been used in a wide field.
Most of such adhesives are used as adhesive articles in an embodiment wherein they are applied to one or both surfaces of a substrate, such as paper, a plastic film, a woven fabric, a nonwoven fabric or a foam product. Such adhesive articles are requested to be free from occurrence of troubles, such as peeling, lifting, foaming and blister, in various use environments.
In order to solve such troubles, the acrylic adhesives need to have performance characteristics, such as cohesive force, adhesive strength and stress relaxation property, with a good balance.
The adhesive articles are sometimes used by applying them to not only plain surface but also an adherend having depressions and protrusions on its surface or an adherend having a curved surface or by applying them to a base having flexibility such as a film that is deformed when it is laminated. In such application conditions where stress is applied to the adhesive layer, adhesive performance of higher level is required, and various trials and errors have been made.
For example, it is disclosed in a patent document 1 that an adhesive having a good balance of adhesive strength, cohesive force, heat resistance and stress relaxation property is obtained by allowing the acrylic polymer to have a high molecular weight of 500,000 to 1,500,000.
In a situation such that great stress is applied as in the case of application to a curved surface, however, a satisfactory stress relaxation effect is not obtained by merely allowing the adhesive polymer to have a high molecular weight. In the preparation of a high-molecular weight polymer, further, there are problems that the polymerization operation is difficult and special polymerization conditions and apparatus become necessary. Moreover, because the high-molecular weight polymer has an extremely high viscosity, the polymer needs to be diluted with a large amount of an organic solvent in order to control the polymer to have a viscosity suitable for the application, so that there are problems that the cost is high and drying of a solvent requires great energy.
In a patent document 2, there is disclosed an adhesive obtained by blending an acrylic polymer having a highly reactive hydroxyl group in a polymer and having a weight-average molecular weight of 5,000 to 300,000 with a polyfunctional isocyanate compound containing bifunctional isocyanate as a main body. In this adhesive, by allowing the polymer of a relatively low-molecular weight to react with the polyfunctional isocyanate compound, the acrylic polymer is linked to perform chain extension (increase of molecular weight). By virtue of this, the high-molecular weight polymer acquires properties such as high cohesive force and stress relaxation property.
In the reaction of the polymer having a functional group in a polymer chain with the polyfunctional monomer, however, gelation reaction attributable to three-dimensional crosslinking takes place simultaneously with the extension of the polymer chain length, and therefore, the same performance as in the case of using the high-molecular weight polymer is not obtained. Moreover, because the reaction of the hydroxyl group with the isocyanate group is liable to be influenced by moisture content in air, there is a problem that it is hard to obtain stable properties depending upon the environmental changes.
In a patent document 3, a polymer having a molecular weight of not more than 300,000, which is obtained by photopolymerization of a macromonomer having a molecular weight of about hundreds to tens of thousands and having a reactive functional group at the molecular end and an acrylic monomer, and an adhesive having high cohesive force and high adhesive strength are disclosed. In this invention, the macromonomer and the adhesive polymer are bonded to take a microphase separation structure, and thereby, high cohesive force and high adhesive strength are obtained in spite of a low-molecular weight.
The usual macromonomer, however, has unstable reactivity, and hence, a free macromonomer which does not react with the monomer remains or an adhesive polymer of a low-molecular weight is formed, and these are liable to bleed from the resulting adhesive. Further, in order to form a microphase separation structure, the macromonomer and the acrylic monomer need to be greatly different from each other in composition. On this account, these monomers need to be selected taking their respective compatibilities into consideration, and not only the selection is difficult but also the application range is narrow. Furthermore, there is a fear that the transparency of the acrylic adhesive is impaired or a problem that the stabilization of the adhesive properties is difficult.
An attempt to use the polymer (macromonomer) having reactivity as above to thereby develop cohesive force or adhesive strength of the high-molecular weight polymer or to thereby obtain other various functions has been made. However, the polymerization reactivity between the macromonomer and other monomers is insufficient, or the molecular weight of the polymer obtained by the polymerization is not increased so much because the molecular weight of the macromonomer is low. Thus, sufficient performance has not been obtained yet.
Usually, the macromonomer is prepared by a polymerization process mainly using ionic polymerization or a chain transfer polymerization process mainly using radical polymerization (e.g., patent document 4). In either preparation process, however, the macromonomer is prepared by two-step reaction wherein a functional group is first formed at the end of a polymer by using an initiator having a functional group or a chain transfer agent and then the polymer having an end functional group is allowed to react with a compound having a group reactive to the functional group and having a polymerizable unsaturated group to perform addition of the polymerizable unsaturated group to the end of the polymer. Therefore, these preparation processes are complicated and undesirable.
Further, the macromonomer having an end unsaturated group obtained by such a two-step reaction has a problem of poor reactivity in the polymerization reaction with other monomers. Furthermore, there are many problems relating to qualities such that unreacted end functional groups remain in the reaction for the addition of the polymerizable unsaturated group and a by-product due to a side reaction is present.
In recent years, a process for preparing a macromonomer by a continuous polymerization reaction wherein a vinyl monomer is continuously fed to a reaction vessel under high temperature conditions to polymerize the monomer has been proposed (see patent document 5). In this process, an excellent macromonomer having high reactivity and containing a small amount of a by-product is obtained by the reaction of one-step. However, the reaction temperature is an extremely high temperature of 180 to 500° C. Therefore, the molecular weight of the reactive polymer becomes about thousands to tens of thousands, and a polymer obtained by copolymerization of this macromonomer and other monomers does not acquire a sufficient molecular weight. Moreover, because the reaction is carried out under high temperature conditions, there is a problem that a special apparatus becomes necessary.
Patent document 1: Japanese Patent laid-Open Publication No. H01(1989)-178567
Patent document 2: Japanese Patent laid-Open Publication No. H09(1997)-235539
Patent document 3: Japanese Patent laid-Open Publication No. H04(1992)-366103
Patent document 4: Japanese Patent laid-Open Publication No. 560(1985)-133007
Patent document 5: National Publication of International Patent No. 2001-512753
Accordingly, it is an object of the present invention to provide an adhesive composition which uses an end-reactive acrylic polymer having a double bond at the end, having a high molecular weight and having high polymerization reactivity and is thereby employable as an adhesive capable of stably keeping a bonded state for a long period of time.
It is another object of the present invention to provide an adhesive sheet in which the above adhesive composition is used for an adhesive layer and an adhesive using the adhesive composition.